In an electromagnetic actuator for operating a driven member at least one electromagnet is provided which has a yoke body and with which a back-and-forth movable armature plate is associated. When the electromagnet is energized, the armature plate is moved against the force of a resetting spring from a first switching position into a second switching position in which the armature plate engages the pole face of the yoke body. If the electromagnet is de-energized, the armature plate is returned to its first switching position under the effect of the resetting force of the resetting spring.
For certain applications, for example, for operating an engine valve of a piston-type internal-combustion engine where the engine valve constitutes the driven member, two spaced electromagnets are provided whose pole faces are oriented towards one another and between which the armature plate, attached to a guide bar, is movable back and forth against the force of resetting springs. When the electromagnets are de-energized, the armature plate assumes a mid position between the two electromagnets. If the electromagnets are alternatingly energized, the armature plate moves from a first switching position which is defined by its engagement with the pole face of one electromagnet, into its second switching position after de-energization of the last-named, momentarily holding electromagnet and energization of the other, momentarily capturing electromagnet. The second switching position of the armature plate is defined by its engagement with the pole face of the other, capturing electromagnet. By means of a suitable control of current supply in an alternating manner, the armature plate and thus the driven member may again be moved into the first switching position.
In the earlier-noted mode of application for operating an engine valve in a piston engine not only a high switching frequency is obtained but, based on the relatively large stroke of the engine valve, the armature has an oscillating path (stroke) of significant length. Despite the resetting force of the resetting spring arranged at the capturing electromagnet and acting on the armature plate during its approach to the pole face of the capturing electromagnet, the net attracting force on the armature plate increases exponentially, since, as the distance between the armature plate and the pole face of the capturing electromagnet decreases, the electromagnetic force increases exponentially, while the opposing spring force increases only linearly. As a result, the armature plate is increasingly accelerated until it impacts on the pole face of the capturing electromagnet. The resulting high impact velocity leads to a significant noise generation and may also lead to disadvantageous rebounding phenomena.
Conventional pneumatic dampening arrangements for braking the armature, such as disclosed, for example, in published European Patent Application 0 870 906, not only require a significant structural height; also, they are not capable of delivering the required dampening force with acceptable frictional values.
By means of controlling the current supply of the momentary capturing electromagnet, it is feasible to change the magnetic force during the approaching phase of the armature plate in such a manner that the armature plate arrives into contact with the pole face with a reduced speed. It is, to be sure, in principle possible to change, by controlling the current supply of the capturing electromagnet, the magnetic force in such a manner that the armature plate arrives with a "zero" impact velocity in contact with the pole face of the capturing electromagnet. Because of stochastic oscillations in the system, however, a reliable, secure capturing and holding of the armature plate is not feasible. Thus, for reasons of operational reliability the current supply of the capturing electromagnet must be set in such a manner that the armature plate arrives in contact with the pole face with a reduced, but significant residual speed. While rebound phenomena are in this manner prevented, noise generation nevertheless persists. By means of the above-described reduction of the impact velocity, however, significantly reduced dampening forces are required.